1. Field of the Invention
The present invention relates to a method for forming an insulating film. More specifically, it relates to a method for forming a coating type insulating film having Sixe2x80x94H bonding. The coating type insulating film includes an inorganic SOG (Spin On Glass) film which is particularly suitable as an insulating film, e.g., an interlayer insulating film and a protective film, in semiconductor devices. The present invention can prevent the generation of microscopic projections on the surface of the coating type insulating film having the Sixe2x80x94H bonding by calcining and exposing a resin film to form the insulating film under reduced pressure for converting the resin film into a ceramic film.
2. Description of Related Art
Inorganic coating type insulating films having Sixe2x80x94H bonding have been conventionally provided by coating, baking and calcination. Specifically, a resin film is formed (coated) to cover a surface of a semiconductor substrate by spin coating or the like. Then, a thermal treatment (baking) is performed in an atmosphere of an inert gas such as N2, followed by calcination in an atmospheric pressure oven or a reduced pressure oven. Thus, an insulating film is formed.
According to such a method, an inorganic SOG film having the Sixe2x80x94H bonding and a thickness of 1 xcexcm or more can be obtained without cracks. The thus formed inorganic SOG film (silicon oxide film) is suitable as an insulating film, such as an interlayer insulating film, a protective film and the like, in a semiconductor device.
A method of forming multilayer wirings according to a prior art technique is shown in FIGS. 3(a) to 3(d).
In the step shown in FIG. 3(a), a wiring layer 16 is formed on an insulating film 14 of silicon oxide or the like which covers an entire surface of a semiconductor substrate 12, and then a solution prepared by dissolving a hydrogen silsesquioxane resin into MIBK (methyl isobutyl keton) is spin-coated to form a resin film 18 which alleviates level difference due to the insulating film 14 and the wiring layer 16.
In the step shown in FIG. 3(b), the resin film 18 is thermally treated under an inert gas atmosphere to convert the resin film 18 into a pre-ceramic silicon oxide film 18a. Next, the silicon oxide film 18a is thermally treated in a mixed atmosphere Of O2 gas and an inert gas to convert the silicon oxide film 18a into a ceramic silicon oxide film 18b. At this time, a microscopic projection 18c of about 0.1 xcexcm in diameter is generated on the surface of the ceramic silicon oxide film 18b. 
In the step shown in FIG. 3(c), an insulating film 20 made of silicon oxide is formed by plasma CVD method to cover the ceramic silicon oxide film 18b. Then, the insulating film is planarized by chemical mechanical polishing (CMP) method.
One of drawbacks accompanying the thus generated microscopic projection 18c is that a precise contact hole cannot be formed as shown in FIG. 3(d). According to the above-described conventional technique, the yield of the formation of wirings is reduced since the microscopic projection of about 0.1 xcexcm in diameter is generated on the surface of the ceramic inorganic SOG film having the Sixe2x80x94H bonding.
In the step shown in FIG. 3(d), a contact hole 24 extending to the wiring layer 16 is formed by s elective dry etching (anisotropic etching) using a resist layer 22 as a mask. Thereafter, an ashing treatment is performed to remove the resist layer 22, followed by washing. During this step, the microscopic projection 18c and its neighborhood are over-etched since they are characteristically weak against the etching. As a result, a concave portion 25 is generated on the sidewall of the contact hole 24.
Then, a wiring material layer is formed on the surface of the substrate and patterned to form a wiring layer contacting the wiring layer 16. The coverage of the thus formed wiring layer is deteriorated at the concave portion 25 in the contact hole 24, which reduces reliability.
To solve the above-mentioned drawback, Japanese Unexamined Patent Publication No. Hei 9 (1997)-283515 discloses a method of preventing the generation of the microscopic projection on the surface of the silicon oxide film by thermally treating a pre-ceramic silicon oxide film for converting into a ceramic film in an atmosphere containing O2 and N2 at a temperature of 250xc2x0 C. to 400xc2x0 C.
However, even though the above method is utilized, mechanical strength of the ceramic silicon oxide film is unsatisfactory. Further, when a passivation film is formed after the calcination for forming an SOG film, silane type gas (Sixe2x80x94H) eliminated from the SOG film is confined between the SOG film and the passivation film, which induces the film exfoliation.
Further, if the calcination for forming the SOG film is performed in a reduced pressure oven, the silane type gas (Sixe2x80x94H) is excessively eliminated during calcination, which renders the SOG film fragile.
An object of the present invention is to provide a novel method for forming a silicon oxide film, which is capable of preventing the generation of microscopic projections without reducing the mechanical strength of the silicon oxide film.
According to the method for forming the insulating film of the present invention, a precursor film of a coating type insulating film having Sixe2x80x94H bonding is coated, the precursor film is calcined in an atmosphere containing at least one of an inert gas and oxygen gas for converting it into a ceramic film as the insulating film, and then the ceramic film is cooled under reduced pressure lower than that for the calcination.
The calcining atmosphere can be selected from an inert gas atmosphere, an oxygen gas atmosphere and a mixed atmosphere containing the two gases, depending on the kind of precursor film of the insulating film, material constituting the insulating film and the like.
According to the method for forming the insulating film of the present invention, after the calcination of the precursor film, the pressure is reduced preferably to 1xc3x9710xe2x88x921 Pa or more to 1xc3x9710+3 Pa or less.
Further, according to the method for forming the insulating film of the present invention, after the calcination of the precursor film, the temperature is preferably lowered to 200xc2x0 C. to 350xc2x0 C. under the reduced pressure.
Still further, according to the method for forming the insulating film of the present invention, the pressure at the calcination of the precursor film for converting it into the ceramic film is desirably 0.07xc3x9710+6 Pa or more to 0.13xc3x9710+6 Pa or less.
The present invention can be applied to a method of forming an inorganic SOG film having the Sixe2x80x94H bonding which comprises the steps of performing a first thermal treatment for converting a precursor film of the inorganic SOG film into a pre-ceramic film in an atmosphere containing at least one of an inert gas and oxygen gas, performing a second thermal treatment for calcining the pre-ceramic film into a ceramic film in an atmosphere containing at least one of an inert gas and oxygen gas, and reducing pressure after the calcination by the thermal treatment. The film characteristics of the resulting film can be stabilized by performing the step of forming the pre-ceramic precursor film.
According to the present invention, the generation of the microscopic projections can be prevented because the pressure is reduced after the calcination. The lower limit of the pressure is set to 1xc3x9710xe2x88x921 Pa because if the pressure reduced after the calcination is less than 1xc3x9710xe2x88x921 Pa, the mechanical strength of the resulting film falls behind that of the insulating film formed under atmospheric pressure. In the method of the present invention, the temperature for converting the precursor film into the pre-ceramic film is preferably in the range of 250xc2x0 C. to 400xc2x0 C. Within the temperature range, the precursor film can be converted into the pre-ceramic film by about 1 minute heating.
In the method of the present invention, the thermal treatment for forming the pre-ceramic film can be omitted so that a hydrogen silsesquioxane resin film (precursor film), which serves as the precursor film, can be formed into a ceramic silicon oxide film by one thermal treatment. In this case, the thermal treatment can be performed in an atmosphere containing at least one of an inert gas and oxygen gas at a temperature ranging from 250xc2x0 C. to 400xc2x0 C. As a result, the ceramic silicon oxide film having desired mechanical strength can be obtained without the microscopic projection.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.